1. Field of the Invention
The present invention relates to an optical reader which reads information on an original document by focusing an image on the original document to a photoelectric conversion element such as an image sensor with a focusing optical system, in particular, to an optical reader, which is suitable for a digital image reader, a digital copier and the like, and is developed in connection with an illumination unit illuminating an original document upon reading, an image reader, and an image forming device.
2. Description of the Related Art
FIG. 10 illustrates one example of a general image reader. In the image reader illustrated in FIG. 10, an original document 2 on which an image to be read is depicted is placed on a contact glass 1 of a flat platen. An illumination unit 3 is disposed under the contact glass 1. The illumination unit 3 includes a cylindrical lamp 3A, which extends in the direction orthogonal to FIG. 10, such as a slender xenon lamp or a halogen lamp, and a slender band-like reflector 3B, which reflects light from the lamp 3A, and condenses the light in the direction orthogonal to FIG. 10. This illumination unit 3 illuminates a long band-like portion of the original document 2 in the direction orthogonal to FIG. 10.
In this case, the light reflected by the illuminated portion of the original document 2 with an image is sequentially reflected by five reflection mirrors in order from M1 to M5. A reduced image on the original document 2 is focused on an imaging area of a line sensor 6 as a photoelectric conversion element. The illumination unit 3, the reflection mirrors M1-M5, an image reading lens 5 and the line sensor 6 constitute one unit as an optical reader 7. This optical reader 7 drives in the arrow direction (right side in FIG. 10) by a driving unit (not shown), so as to be displaced to a position illustrated by the dotted lines. The entire information on the original document 2 is thereby read.
The line sensor 6 which images the image on the original document 2 is a three-line color sensor such as a three-line CCD image sensor in which photoelectric conversion elements 6A, 6B, 6C each having R (red), G (green) and B (blue) filters as a color separation section are arranged in three lines on one chip. The line sensor 6 which is the three-line color sensor converts the image on the original document 2 into signals according to the illumination scanning of the original document 2. The color image of the original document 2 is thereby separated into three colors, R, G, B, to be read.
When an image reader uses this optical reader 7, the color separation section (for example, R, G and B filters provided in the photoelectric conversion elements 6A, 6B, 6C) is disposed in the focusing optical path of the image reading lens 5. Therefore, a full-color image can be read.
In addition, as a color separation method, a method of separating into respective colors, R, G, B by selectively inserting a color separation prism or a filter between the image reading lens 5 and the line sensor 6, and a method of separating into respective colors by illuminating the original document 2 by sequentially lighting respective light sources of R, G, B can be used, in addition to a method of using the photoelectric conversion elements 6A, 6B, 6C each having R, G and B filters.
There may be a case which reads information on an original document as a monochrome image without disposing a color separation section in a focusing optical path.
In this case, the five reflection mirrors M1-M5 are used, but the number of reflection mirrors is not limited to five.
FIG. 11 illustrates one example of an image forming device having the optical reader 7. The image forming device illustrated in FIG. 11 includes an image reader 200 disposed in the upper portion of the image forming device, and an image forming unit disposed under the image reader 200. The image reader 200 includes the optical reader 7 illustrated in FIG. 10. The reference numbers used in FIG. 10 are also used for the portions in FIG. 11 corresponding to the portions in FIG. 10.
The image signals output from the three-line sensor 6 of R, G, B which is an imaging section of the image reader 200 are sent to an image processor 120 of the image forming unit, and are processed in the image processor 120. In this case, these signals are converted into signals for writing an electrostatic latent image on a photoreceptor by each color, yellow (Y), magenta (M), cyan (C), and black (K).
The image forming unit includes a cylindrical photoreceptor 1100 as an electrostatic latent image carrier. The photoreceptor 1100 includes therearound a charging roller 1110 as a charging section, a turret-type developer 130, a transfer belt 1140 and a cleaner 1150. As the charging section, a corona charger can be used instead of the charging roller 1110.
An optical scanner 1170 receives the writing signals from the image processor 1200, and an electrostatic latent image is written on the photoreceptor 1100 by the scanning of an optical beam. In this case, the optical scanner 1170 writes an electrostatic latent image on the outer circumference face of the photoreceptor 1100 between the charging roller 1110 and the developer 1130.
The image forming unit of the image forming device includes the lower portion thereof a fusing unit 1160, a cassette 1180, a resistor roller pair 1190, a paper feeding roller 1220 and a tray 1201. A path for transfer paper S as recording media is formed under the photoreceptor 1100.
When forming an image, the photoreceptor 1110 rotates at a constant speed in the clockwise direction in FIG. 11, and the outer circumference surface of the photoreceptor 1100 is uniformly charged by the charging roller 1110. The outer circumference surface of the photoreceptor 1100 is exposed by the light beam emitted from the optical scanner 1170, and an electrostatic latent image is formed on the outer circumference surface of the photoreceptor 1100. The formed electrostatic latent image is a so-called negative latent image in which an image portion is exposed.
The writing of the electrostatic latent image is conduced in order from a yellow (Y) image, a magenta (M) image, a cyan (C) image and a black (K) image according to the rotation of the photoreceptor 1100. The electrostatic latent image formed on the outer circumference surface of the photoreceptor 1100 is reversely-developed by each developer unit Y, M, C, K of a turret-type developer station 1130 such as a cylinder in a revolver, more particularly, a Y developer unit which develops by yellow toner, a M developer unit which develops by magenta (M) toner, a C developer unit which develops by cyan toner, and a K developer unit which develops by black (K) toner, so as to be visualized as a positive image. The toner image of each color formed on the outer circumference surface of the photoreceptor 1100 is sequentially transferred on the transfer belt 1140 by a transfer voltage application roller 114A. A color image in which the above toner image of each color is overlapped on the transfer belt 1140 is then obtained.
The cassette 1180 in which transfer paper S is housed is detachably attached to the body of the image forming device. In a state in which the cassette 1180 is attached to the image forming device as illustrated in FIG. 11, the top sheet of the transfer paper S housed in the cassette 1180 is fed to a paper feeding path by the paper feeding roller 1220, and the leading end portion of the sheet is caught by the resist roller pair 1190.
The resist roller pair 1190 sends the transfer sheet S to the transfer unit in a timing in which the color image formed on the transfer belt 1140 moves to the transfer position. The transfer sheet S fed to the transfer unit is overlapped to the color image in the transfer unit, and is pressed to the color image by the transfer roller 114B. The color image is thereby electrostatically transferred onto the transfer paper S.
The transfer sheet S onto which the color image is transferred is sent to the fusing unit 1160, so that the color image is fused. Then, the transfer sheet S is guided by a guide unit (not shown) which guides the transfer sheet S, is moved on the transfer path, and is discharged on the tray 1210 by a paper discharging roller pair (not shown).
The outer circumference surface of the photoreceptor 1100 is cleaned by the cleaning unit 1150 every time the toner image of each color is transferred, so that the residual toner, paper power and the like are eliminated.
When forming a monochrome image without forming a full-color image, a known image forming unit for a single color image is used instead of the above image forming unit.
A cylindrical lamp such as a xenon lamp or a halogen lamp generally for use in the illumination unit of the optical reader 7 illustrated in FIG. 10 has large power consumption and also large heat generation, so that the temperature in the entire device is increased. In the image reader using the optical reader including the illumination unit, the imaging optical system, and the photoelectric conversion element as one unit illustrated in FIG. 10, since the sealing performance of the optical reader is high, the increase in the temperature in the optical reader 7 becomes larger by the heat generation of the cylindrical lamp in the illumination unit. If the increase in the temperature in the optical reader 7 becomes large by the heat generation of the cylindrical lamp in the illumination unit, the conjugate relation of the optical system is disturbed, and the focusing is not conducted to the photoelectric conversion element (an image is not focused onto the photoelectric conversion element). Consequently, a preferable image is not obtained.
In order to downsize the image reader, a cold cathode fluorescent lamp (hereinafter, referred to as a CCFL) is often used instead of using the above xenon lamp or halogen lamp. The CCFL requires time to stabilize the illumination amount, so that it takes a long preparation time for reading an original document. The CCFL contains mercury (Hg) having a problem from an environmental aspect regarding recycling and the like.
For this reason, an illumination unit using an LED (light-emitting diode) light source, which has low power consumption, small heat generation, and a long operating life, and an advantage from an environment aspect regarding recycling and the like, has attracted attention as a new illumination light source instead of the above lamps.
As the above illumination unit using an LED, an illumination unit in which LEDs are arranged at given intervals is disclosed in Patent Document 1 (JP2006-67551A), Patent Document 2 (JP2004-157213A), Patent Document 3 (JP2005-27082A), Patent Document 4 (JP2002-142082A), Patent Document 5 (JP2005-241681A), and Patent Document 6 (JP2006-42016A).
For example, in Patent Document 1, a method of effectively illuminating a surface of an original document by controlling the spread of the light in the arrangement direction of the LEDs in one illumination unit is disclosed. In Patent Document 1, a method of reducing uneven illumination according to the arrangement of the LEDs when arranging the LEDs is disclosed, but this method illuminates an original document only at a certain angle.
In general, when reading an original document having a gap in the direction orthogonal to the scanning direction of an illumination unit (i.e., scanning direction of original document) as a cut-and-paste original document in which a cut original document is pasted to another original document by bond or the like, for example, shadow is created by the light blocked by the gap. Therefore, it is known that an abnormal image is obtained, which generates a black shadow line in the read original information. It is also known that a phenomenon similar to that in the above cut-and-paste original document occurs in the boundary face between the original document and a plate which presses the original document in the end portion of the original document.
As a method of reducing such a phenomenon, it is effective to illuminate an original document from another direction as described in Patent Document 1. However, this method requires at least two illumination units. In addition, a certain length of a light source of the illumination unit is required in order to reduce the uneven illumination. For this reason, as illustrated in FIG. 12, the measurement h1 in the height direction of the illumination unit is increased, so that a space which can not be used for deflecting luminous flux by means of the reflection mirrors in the optical reader 7, i.e., a dead space is increased in size. As a result, the height H and the width W of the optical reader 7 are increased, resulting in the increase in the power consumption and the costs of the device. In order to effectively illuminate, it is necessary to ensure a regular distance from the LED of the illumination light source to the surface of the original document or more. Therefore, it is necessary to increase the size of the optical reader 7 in the normal direction of the surface of the original document.
In contrast, in Patent Documents 2, 3, 4, a method is disclosed of illuminating from the other side with the reflection light of mirrors disposed on the side opposite to the light source.
In the structure described in Patent Document 2, since the LED directly faces a side to be illuminated, the light volume to be reflected by the mirror is reduced. In this case, in the cut-and-paste original document in which the second original document is pasted on the first original document, the generation of the shadow is different between the illumination from the first original document side toward the second original document side and the illumination from the second original document side toward the first original document side. If the cut-and-paste original document is illuminated from one side, the shadow is easily generated compared with a case in which the cut-and-paste original document is illuminated from the other side. More particularly, if an original document includes a pasted portion, shadow is generated according to the direction of the illumination relative to a boundary division of the pasted portion.
In the structure described in Patent Document 3, an LED for direct illumination is disposed different from an LED for reflection illumination. Since a so-called shell-type LED is directly disposed, it is necessary for the illumination unit to be disposed close to the surface of the original document, in order to effectively illuminate the surface of the original document. When disposing the illumination unit close to the surface of the original document, uneven illumination according to the arrangement pitches of the LEDs easily occurs because of the angular dependency of the emission of the LEDs. Therefore, it is necessary to arrange the LEDs at narrow pitches even if a required illuminance is low. For this reason, the illumination efficiency is low.
The illumination unit described in Patent Document 4 has a structure which illuminates a surface of an original document by disposing two reflection planes each of which has a concave face on an LED side. In this structure, similar to the above structure in Patent Document 3, if the LEDs are arranged at wide pitches, uneven illumination in the arrangement direction according to the arrangement intervals of the LEDs occurs on the surface of the original document. Therefore, it is necessary to arrange the LEDs at narrow pitches. On the other hand, if the distance from the LEDs to the reflection plane is increased, the uneven illumination can be reduced. However, such a structure requires a large reflection plane, so that the size of the illumination unit has to be increased.
In addition, a structure which uses a halogen lamp as a light source, and disposes a reflection section for reflecting illumination light in the original document direction between the light source and the original reading surface is described in Patent Document 7 (JP3979741B).
In the structure of Patent Document 7, the halogen lamp is a cylindrical lamp, and the emission intensity of the projection light is even relative to the normal direction of the tube of the halogen lamp HL as illustrated in FIG. 13. For this reason, as illustrated in FIG. 14, if the illumination light is not reflected by reflection planes RS3, RS4 in an optical system around the halogen lamp HL, so as to be deflected, it is difficult for reflection planes RS1, RS2 each of which guides the illumination light to the surface of the original document to effectively guide the illumination light to the reading face of the original document, and the light use efficiency in the original document reading position is extremely deteriorated. In such an illumination optical system, the projection light from the halogen lamp HL is deteriorated in its light volume by the reflection with the peripheral reflection planes. Therefore, the heat generation amount (light volume) and the size of the optical system surrounding the halogen lamp (illumination unit size) have to be increased, in order to obtain sufficient light volume on the original document surface.